Synthesis of 3-Amino-4-substituted Monocyclic ß-Lactams—Important Structural Motifs in Medicinal Chemistry

Monocyclic ß-lactams (azetidin-2-ones) exhibit a wide range of biological activities, the most important of which are antibacterial, anticancer, and cholesterol absorption inhibitory activities. The synthesis of decorated monocyclic ß-lactams is challenging because their ring is highly constrained and consequently reactive, which is also an important determinant of their biological activity. We present the optimized synthesis of orthogonally protected 3-amino-4-substituted monocyclic ß-lactams. Among several possible synthetic approaches, Staudinger cycloaddition proved to be the most promising method for initial ring formation, yielding monocyclic ß-lactams with different substituents at the C-4 position, a phthalimido-protected 3-amino group, and a (dimethoxy)benzyl protected ring nitrogen. Challenging deprotection methods were then investigated. Oxidative cleavage with cerium ammonium nitrate and ammonia-free Birch reduction was found to be most effective for selective removal of ring nitrogen protection. Hydrazine hydrate was used for deprotection of the phthalimido group, and the procedure had to be modified by the addition of HCl in the case of aromatic substituents at the C-4 position. The presented methods and the synthesized 3-amino-4-substituted monocyclic ß-lactam derivatives are an important step toward new ß-lactams with potential pharmacological activities.


Introduction
Ever since Alexander Fleming's serendipitous discovery of the first broad-spectrum ß-lactam antibiotic (i.e., penicillin G) in the late 1920s, ß-lactams arguably remain the single most clinically useful class of antibiotics discovered to date, in some countries making up over 60% of all antibiotic sales [1,2]. Their excellent safety and efficacy profiles and the highly reactive nature of the CO-N bond in the ß-lactam ring have propelled this structural motif in many drug discovery initiatives, besides their primary use as antibacterial agents [3][4][5]. Not only presence of the bicyclic ring system of penicillins is essential for their antibacterial activity, but it can also be replaced by a monocyclic ß-lactam. Appropriately decorated 3-aminoazetidin-2-ones serve as mimics of the D-Ala-D-Ala subunit of the stem peptide in the nascent peptidoglycan. ß-Lactams act as mechanism-based inhibitors of the transpeptidase activity of penicillin-binding proteins (PBPs), thus inhibiting the cross-linking step in peptidoglycan chains, ultimately leading to bacterial cell death [6,7]. Aztreonam, the first clinically approved synthetic monobactam (i.e., N-sulfonated monocyclic ß-lactam) in the 1980s, is still in use worldwide because of its suitable activity against Gram-negative bacteria and ß-lactamase stability [8]. Another approved monocyclic ß-lactam drug in clinical use is ezetimibe, which acts as a cholesterol absorption inhibitor and is used to treat hypercholesterolemia [9,10].

Results and Discussion
Our initial efforts to prepare the desired 3-amino-4-substituted monocyclic ß-lactams through N1-C4 ring closure reactions (e.g., via Mitsunobu cyclization [31] or bromineinduced cyclization [32]) were unproductive. In a subsequent approach, the convergent methodology for the stereoselective synthesis of functionalized β-lactams with a broad substrate scope developed by Staudinger et al. [20] was explored. The preparation of the ketene and imine building blocks required for the Staudinger [2+2] cycloaddition to provide target ß-lactams, as well as our studies on the necessary N-1 and N-3 deprotections, are discussed in more detail below.

Methods for Cyclization of 2-Azetidinone (Monocyclic Beta Lactam Core)
The requisite imines (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) were formed by the condensation of appropriate primary amines and aldehydes in dichloromethane or methanol at room temperature, using anhydrous sodium sulfate as a drying agent (Scheme 1). To demonstrate the possibility of incorporating a variety of different substituents at the C-4 position of the monocyclic ßlactam, we selected several aldehydes from our in-house library of chemicals. The selected amines were previously described in the synthesis of monocyclic ß-lactams. In the case of the more reactive aliphatic aldehydes (15)(16)(17), the condensation reactions were carried out on ice, and the imines were used directly without evaporation of the solvent.
We initially focused our efforts on the preparation of monocyclic ß-lactams using ketenes obtained from t-butylcarbamateor benzylcarbamate-protected glycine and imines derived from aromatic aldehydes to test reactivity in Staudinger model reactions. However, the expected [2+2] cycloaddition products (i.e., 2-azetidinones) were not observed with any of the evaluated carbamates. This may be due to the competing formation of 1,3-oxazin-4-ones, which are highly stable and cannot react further to form 2-azetidinones [33,34]. Therefore, we have elected to use a phthalimido group to protect the glycine-amino group instead. Ketenes, prepared in situ from an acyl chloride with N-phthalimido protecting group (e.g., 18), were prone to undergo the desired cycloadditions (Scheme 2). The reactions proceeded smoothly when the nitrogen of the amino acid residue was protected by substitution of both hydrogen atoms, as in the case of phthalimido-protected glycine. An acyl chloride was added dropwise to a mixture of imine and a base in toluene at 80 • C, and the product formed was easily isolated by precipitation or column chromatography. Because of the instability of ketenes, the order of the addition of the reactants was also an important factor. derived from aromatic aldehydes to test reactivity in Staudinger model reactions. However, the expected [2+2] cycloaddition products (i.e., 2-azetidinones) were not observed with any of the evaluated carbamates. This may be due to the competing formation of 1,3oxazin-4-ones, which are highly stable and cannot react further to form 2-azetidinones [33,34]. Therefore, we have elected to use a phthalimido group to protect the glycineamino group instead. Ketenes, prepared in situ from an acyl chloride with N-phthalimido protecting group (e.g., 18), were prone to undergo the desired cycloadditions (Scheme 2). The reactions proceeded smoothly when the nitrogen of the amino acid residue was protected by substitution of both hydrogen atoms, as in the case of phthalimido-protected glycine. An acyl chloride was added dropwise to a mixture of imine and a base in toluene at 80 °C, and the product formed was easily isolated by precipitation or column chromatography. Because of the instability of ketenes, the order of the addition of the reactants was also an important factor. The imines (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) were mainly obtained by the reactions of aromatic aldehydes (which were substituted by electron-withdrawing groups) with dimethoxybenzylamine or benzylamine. The products of the Staudinger reaction (19)(20)(21)(22)(23)(24)(25)(26)(27) in the case of an aromatic or heterocyclic substituent at the C-4 position of the ring were mainly isolated as cis-isomers; trans-stereoisomers were either not detected or were only present in traces that we could not isolate. Staudinger cycloaddition is a stepwise reaction initiated by the nucleophilic attack of an imine on a ketene, leading to a zwitterionic intermediate, followed by ring closure of this intermediate. Direct ring closure leads to the cis-stereoisomer, while indirect ring closure with further isomerization leads to the trans-stereoisomer. As previously reported in the literature, we found that electron-withdrawing groups on the imine facilitate the progress of the reaction, and electron-donating groups slow down the cyclization. Improved yields and exclusive formation of cis-stereoisomer were obtained with imines bearing aromatic substituents on the imine moiety, compared to imines formed from aliphatic aldehydes, which provided much lower yields and lower diastereoselectivity. The cis-configuration of newly synthesized monocyclic β-lactams was deduced using 1 H NMR coupling constants (J values) of the β-lactam ring hydrogens H-3 and H-4; for cis-β-lactams J 3,4~5 Hz, and for trans-β-lactams J 3,4~2 Hz [21].
Since the removal of the phthalimide protecting group requires relatively harsh conditions, we opted to prepare ß-lactam analogs bearing carbamate protecting groups at the N-3 position instead, which we hoped would be more easily removed. Alternatively, functionalized 2-azetidinones can also be prepared via microwave-assisted coupling of imines with diazoketones, which can be derived from t-butylcarbamateor benzylcarbamateprotected α-amino acids [33]. Such monocyclic ß-lactams are structurally different from analogous derivatives prepared via the previously described acyl chloride method by having an additional methylene unit present at C-3 of the ß-lactam ring (Scheme 3). The monocyclic ß-lactams (32)(33)(34), which were prepared using this methodology, were isolated as trans-isomers, as opposed to the otherwise cis-isomers, which are formed via, e.g., Staudindger synthesis. A significant disadvantage of this method is the preparation of diazoketone (31), as most methods require the use of highly toxic diazomethane or expensive trimethylsilyldiazometane [35].
Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 6 of 26 or heterocyclic substituent at the C-4 position of the ring were mainly isolated as cis-isomers; trans-stereoisomers were either not detected or were only present in traces that we could not isolate. Staudinger cycloaddition is a stepwise reaction initiated by the nucleophilic attack of an imine on a ketene, leading to a zwitterionic intermediate, followed by ring closure of this intermediate. Direct ring closure leads to the cis-stereoisomer, while indirect ring closure with further isomerization leads to the trans-stereoisomer. As previously reported in the literature, we found that electron-withdrawing groups on the imine facilitate the progress of the reaction, and electron-donating groups slow down the cyclization. Improved yields and exclusive formation of cis-stereoisomer were obtained with imines bearing aromatic substituents on the imine moiety, compared to imines formed from aliphatic aldehydes, which provided much lower yields and lower diastereoselectivity. The cis-configuration of newly synthesized monocyclic β-lactams was deduced using 1 H NMR coupling constants (J values) of the β-lactam ring hydrogens H-3 and H-4; for cis-β-lactams J3,4 ~ 5 Hz, and for trans-β-lactams J3,4 ~ 2 Hz [21].
Since the removal of the phthalimide protecting group requires relatively harsh conditions, we opted to prepare ß-lactam analogs bearing carbamate protecting groups at the N-3 position instead, which we hoped would be more easily removed. Alternatively, functionalized 2-azetidinones can also be prepared via microwave-assisted coupling of imines with diazoketones, which can be derived from t-butylcarbamate-or benzylcarbamate-protected α-amino acids [33]. Such monocyclic ß-lactams are structurally different from analogous derivatives prepared via the previously described acyl chloride method by having an additional methylene unit present at C-3 of the ß-lactam ring (Scheme 3). The monocyclic ß-lactams (32)(33)(34), which were prepared using this methodology, were isolated as trans-isomers, as opposed to the otherwise cis-isomers, which are formed via, e.g., Staudindger synthesis. A significant disadvantage of this method is the preparation of diazoketone (31), as most methods require the use of highly toxic diazomethane or expensive trimethylsilyldiazometane [35].   Finally, another convenient method was used to synthesize the ß-lactam ring from t-butylcarbamate or benzylcarbamate-protected α-amino acids by Staudinger reaction. [36] Cycloaddition was carried out with the ketenes derived from the mixed anhydride at −70 • C in dry tetrahydrofuran (35)(36)(37)Scheme 4). Again, the cis isomer was a major product but with lower yields, which could not be improved by changing the addition order of the reactants. Finally, another convenient method was used to synthesize the ß-lactam ring from tbutylcarbamate or benzylcarbamate-protected α-amino acids by Staudinger reaction. [36] Cycloaddition was carried out with the ketenes derived from the mixed anhydride at −70 °C in dry tetrahydrofuran (35)(36)(37)Scheme 4). Again, the cis isomer was a major product but with lower yields, which could not be improved by changing the addition order of the reactants. Scheme 4. General reaction scheme for Staudinger [2+2] cyclocondensation starting from ketene, generated from mixed anhydride, and Schiff base. Reagents and reaction conditions: 1. ClCOOEt, Et3N, THF; 2. Et3N, DCM.

Deprotection of C3-NH2 Protecting Group
Since the phthalimide (Phth) moiety is the most commonly used amino protecting group in the ß-lactam ring cyclization reaction (because the cyclization of such Phth-protected ketenes proceeds in high yields), we wanted to optimize the conditions for its deprotection. However, the deprotection methods are quite harsh as they usually involve the use of a very strong base, such as hydrazine hydrate. A variety of Phth deprotection reagents were surveyed, including ethylenediamine, ethanolamine, methylhydrazine, and hydrazine hydrate. [37] The highest product isolated yield was obtained when hydrazine hydrate was used (in contrast, the yield was considerably lower with ethylenediamine and ethanolamine, which are also milder reagents). In the case of methyl hydrazine, the reaction was very slow, even with a high excess of reagent used.
The problem, which has not been described in the literature, is that the reaction of the phthalimide-protected monocyclic ß-lactams (39)(40) with the hydrazine hydrate very likely stops after 1 h because a salt forms with the hydrazine (38). Removal of the excess hydrazine and further addition of a few drops of concentrated hydrochloric acid breaks down the salt formed (Scheme 5). Once the HCl is removed, deprotection of the phthalimide group can continue, and the deprotected ß-lactams with the free amine group at C-3 can be isolated in high yields. Again, in the case of aliphatic substituents (41), which

Deprotection of C3-NH 2 Protecting Group
Since the phthalimide (Phth) moiety is the most commonly used amino protecting group in the ß-lactam ring cyclization reaction (because the cyclization of such Phthprotected ketenes proceeds in high yields), we wanted to optimize the conditions for its deprotection. However, the deprotection methods are quite harsh as they usually involve the use of a very strong base, such as hydrazine hydrate. A variety of Phth deprotection reagents were surveyed, including ethylenediamine, ethanolamine, methylhydrazine, and hydrazine hydrate. [37] The highest product isolated yield was obtained when hydrazine hydrate was used (in contrast, the yield was considerably lower with ethylenediamine and ethanolamine, which are also milder reagents). In the case of methyl hydrazine, the reaction was very slow, even with a high excess of reagent used.
The problem, which has not been described in the literature, is that the reaction of the phthalimide-protected monocyclic ß-lactams (39)(40) with the hydrazine hydrate very likely stops after 1 h because a salt forms with the hydrazine (38). Removal of the excess hydrazine and further addition of a few drops of concentrated hydrochloric acid breaks down the salt formed (Scheme 5). Once the HCl is removed, deprotection of the phthalimide group can continue, and the deprotected ß-lactams with the free amine group at C-3 can be isolated in high yields. Again, in the case of aliphatic substituents (41), which have an electron donor electronic effect than aromatic ones, deprotection with hydrazine hydrate proceeded rapidly and without any adjustments.

Deprotection of N1 Protecting Group
With the optimized conditions for the [2+2] cycloaddition in hand (Scheme 2), we moved our attention to the identification of the most optimal protecting group for the lactam amide nitrogen (i.e., N-1), that would (i) favor the cyclization, and (ii) be easily removable at the end. The preparation of target monocyclic ß-lactams was highly dependent on the success of N-1 deprotection. [38] The deprotection conditions had to be harsh enough to remove the protecting group without concurrent opening of the highly sensitive ß-lactam ring.
In the initial studies, we prepared a small set of N-benzyl ß-lactams (19, 23, 32-33) because we expected to be able to remove this protecting group easily with catalytic hydrogenation. Unfortunately, none of the traditional catalysts and hydrogen sources employed (e.g., Pd/C, Pd(OH)2 with cyclohexene) yielded any product. We have also attempted the aforementioned catalytic hydrogenation under elevated pressure (30 bar); LC-MS and NMR analyses of the reaction mixtures revealed that under these conditions, the N-1 benzyl group was cleaved, but the product yield was too low to enable the isolation and purification of the desired compounds (Table 1)

Deprotection of N1 Protecting Group
With the optimized conditions for the [2+2] cycloaddition in hand (Scheme 2), we moved our attention to the identification of the most optimal protecting group for the lactam amide nitrogen (i.e., N-1), that would (i) favor the cyclization, and (ii) be easily removable at the end. The preparation of target monocyclic ß-lactams was highly dependent on the success of N-1 deprotection. [38] The deprotection conditions had to be harsh enough to remove the protecting group without concurrent opening of the highly sensitive ßlactam ring.
In the initial studies, we prepared a small set of N-benzyl ß-lactams (19, 23, 32-33) because we expected to be able to remove this protecting group easily with catalytic hydrogenation. Unfortunately, none of the traditional catalysts and hydrogen sources employed (e.g., Pd/C, Pd(OH) 2 with cyclohexene) yielded any product. We have also attempted the aforementioned catalytic hydrogenation under elevated pressure (30 bar); LC-MS and NMR analyses of the reaction mixtures revealed that under these conditions, the N-1 benzyl group was cleaved, but the product yield was too low to enable the isolation and purification of the desired compounds (Table 1).
Since deprotection of the benzyl group proved highly problematic, we prepared some ß-lactams with dimethoxybenzyl protecting group at the N-1 position (20-22, 29-30, 35-37). There are several published procedures for removing the para-methoxybenzyl or di-methoxybenzyl group from the amide nitrogen. The procedures that we investigated are summarized in Table 1. First, we attempted to treat N-1-dimethoxybenzyl ß-lactam with strong acids, such as p-toluenesulfonic acid and trifluoroacetic acid (at 60 • C), but this yielded only starting material, and a side product that we assumed (based on NMR) was an opened ß-lactam ring [39]. Next, we have attempted to deprotect N-1 via oxidative cleavage of the dimethoxybenzyl protecting group. Several procedures using persulfate salts (e.g., potassium and/or ammonium persulfate, under various conditions including heating and acid addition [40,41]), which are known to be strong oxidizing agents, provided poor yields and numerous side products, making the isolation of the desired product by column chromatography extremely challenging. A process commonly used to deprotect lactam nitrogen in the literature was Birch reduction [42]. Since the standard process requires the use of toxic liquid ammonia and is often very time consuming, we turned our attention to the more recently published ammonia-free Birch reduction [43]. While the reaction under ammonia-free Birch reduction conditions provided no desired product in the case of dimethoxybenzyl, and benzyl ßlactam derivates with aromatic C-4 substituents, an opened ß-lactam ring with eliminated phthalimide group has been isolated as an exclusive product. In the case of the trifluorophenyl group (42), the fluorine atoms were exchanged for hydrogen (Scheme 6). The situation was quite different for compounds with aliphatic substituents, where the above deprotection could be performed in excellent yields and with almost no side products detected (Table 1).
Finally, the best and most reliable deprotection approach was achieved by using a milder oxidant, cerium ammonium nitrate (46-50) [44]. Oxidative cleavage of N-dimethoxybenzyl protection with cerium ammonium nitrate in aqueous acetonitrile was achieved at the temperature of −10 • C, with the minimum formation of side product (<10%; e.g., compound 45, Scheme 7). We found that the absence of atmospheric oxygen and the water/acetonitrile ratio were important factors in the amount of side product formed. Various relative amounts of acetonitrile/water were tried (from the ratio MeCN/H 2 O = 2:1 to 1:3), with the proportion of product varying from 14% to 52%. The best yield was obtained with a 1:1 ratio of water: acetonitrile, with minimal formation of oxidized, non-deprotected side products observed. However, oxidative dimethoxybenzyl cleavage with cerium ammonium nitrate was unsuccessful for monocyclic ß-lactams that had aliphatic substituents at the C-4 position.
cleavage with cerium ammonium nitrate was unsuccessful for monocyclic ß-lactams that had aliphatic substituents at the C-4 position. Finally, as an example of the synthetic potential of the methods described in this manuscript, we have prepared a fully deprotected 3-amino-4-substituted azetidin-2-one 54 (Scheme 8). The first step after cyclization was the cleavage of the phthalimide protecting group, as this requires the harshest conditions for deprotection, and the ß-lactam ring still protected at the lactam nitrogen is the most stable. Since oxidative cleavage of the dimethoxybenzyl protecting group with a free amino group at the C-3 position was not Finally, as an example of the synthetic potential of the methods described in this manuscript, we have prepared a fully deprotected 3-amino-4-substituted azetidin-2-one 54 (Scheme 8). The first step after cyclization was the cleavage of the phthalimide protecting group, as this requires the harshest conditions for deprotection, and the ß-lactam ring still protected at the lactam nitrogen is the most stable. Since oxidative cleavage of the dimethoxybenzyl protecting group with a free amino group at the C-3 position was not Finally, as an example of the synthetic potential of the methods described in this manuscript, we have prepared a fully deprotected 3-amino-4-substituted azetidin-2-one 54 (Scheme 8). The first step after cyclization was the cleavage of the phthalimide protecting group, as this requires the harshest conditions for deprotection, and the ß-lactam ring still protected at the lactam nitrogen is the most stable. Since oxidative cleavage of the dimethoxybenzyl protecting group with a free amino group at the C-3 position was not possible, we protected it again with a t-butyl carbamate protecting group that is stable to oxidation. For this purpose, we used di-tert-butyl dicarbonate with triethylamine in dichloromethane. After successful conversion of the phthalimide to the t-butyl carbamate protecting group (43, 51-53, shown in Scheme 8), we used cerium ammonium nitrate to remove the dimethoxybenzyl protecting group from the ring nitrogen or ammonia-free Birch reduction in case of aliphatic substituent on C-4 position. Deprotection of the t-butyl carbamate protecting group with hydrochloric acid (4N HCl/dioxane) failed and resulted in the isolation of an opened monocyclic ß-lactam ring. However, the use of trifluoroacetic acid with anisole as a scavenger agent removed the Boc-protecting group in high yield (Scheme 8, compound 54).
possible, we protected it again with a t-butyl carbamate protecting group that is stable to oxidation. For this purpose, we used di-tert-butyl dicarbonate with triethylamine in dichloromethane. After successful conversion of the phthalimide to the t-butyl carbamate protecting group (43,(51)(52)(53), shown in Scheme 8), we used cerium ammonium nitrate to remove the dimethoxybenzyl protecting group from the ring nitrogen or ammonia-free Birch reduction in case of aliphatic substituent on C-4 position. Deprotection of the t-butyl carbamate protecting group with hydrochloric acid (4N HCl/dioxane) failed and resulted in the isolation of an opened monocyclic ß-lactam ring. However, the use of trifluoroacetic acid with anisole as a scavenger agent removed the Boc-protecting group in high yield (Scheme 8, compound 54).

Conclusions
Using Staudinger [2+2] cycloaddition, we successfully synthesized a series of diprotected monocyclic ß-lactams with different substituents at the C-4 position. These initial ß-lactams had phthalimido-protected 3-amino group and dimethoxybenzyl protected ring nitrogen (N-1). Through an extensive study of previously published methods and their subsequent optimization, we have achieved the selective deprotection of both protecting groups in high yield. Oxidative cleavage with cerium ammonium nitrate selectively removed the N-1 protecting group when the aromatic substituents were at the C-4 position, while ammonia-free Birch reduction provided the highest yields for compounds with aliphatic C-4 substituents. For the removal of the phthalimido group, hydrazine hydrate provided the best yield, but in the case of aromatic substituents at the C-4 position, synthetic modification by HCl addition was required. The presented methods and the synthesized protected and partially deprotected 3-amino-4-substituted monocyclic ß-lactams are an important step toward new ß-lactams with potential pharmacological activities.

Conclusions
Using Staudinger [2+2] cycloaddition, we successfully synthesized a series of diprotected monocyclic ß-lactams with different substituents at the C-4 position. These initial ß-lactams had phthalimido-protected 3-amino group and dimethoxybenzyl protected ring nitrogen (N-1). Through an extensive study of previously published methods and their subsequent optimization, we have achieved the selective deprotection of both protecting groups in high yield. Oxidative cleavage with cerium ammonium nitrate selectively removed the N-1 protecting group when the aromatic substituents were at the C-4 position, while ammonia-free Birch reduction provided the highest yields for compounds with aliphatic C-4 substituents. For the removal of the phthalimido group, hydrazine hydrate provided the best yield, but in the case of aromatic substituents at the C-4 position, synthetic modification by HCl addition was required. The presented methods and the synthesized protected and partially deprotected 3-amino-4-substituted monocyclic ß-lactams are an important step toward new ß-lactams with potential pharmacological activities.

Chemistry and Chemical Characterization of Compounds
Unless otherwise stated, all reactions were carried out under argon atmosphere in flame-dried glassware. Chemicals and solvents were obtained from commercial sources (Sigma-Aldrich, Acros Organics, TCI Europe, fluorochem, and Apollo Sci) and were used as supplied. Dry solvents were prepared by distillation from CaH 2 (CH 2 Cl 2 ) or from a mixture of sodium and benzophenone (tetrahydrofuran). Other solvents (dimethylformamide, toluene, methanol, and CH 3 CN) were used directly from anhydrous Aldrich Sure/Seal bottles. Evaporation of the solvent was carried out under reduced pressure. Reactions were monitored by thin-layer chromatography (TLC) on silica gel aluminum plates (Merck DC Fertigplatten Kieselgel 60 GF254), visualized under UV light (254 nm), and stained with appropriate TLC stains for detection (ninhydrin, dinitrophenylhydrazine, and phosphomolybdic acid). The products were purified by flash column chromatography performed on Merck silica gel 60 (mesh size, 70-230) using the indicated solvents. Yields are reported for the purified products. 1 H NMR and 13 C NMR spectra were recorded at 295 K using a Bruker Avance III NMR spectrometer equipped with a Broadband decoupling inverse 1 H probe, at 400 MHz and 100 MHz, respectively. Chemical shifts (δ) are given in parts per million (ppm) and refer to tetramethylsilane (TMS) as an internal standard. The coupling constants (J) are given in Hertz (Hz), and the splitting patterns are reported as: s, singlet; br s, broad singlet; d, doublet; dd, double doublet; t, triplet, and m, multiplet. Mass spectra were recorded using an ADVION Expres-sion CMSL mass spectrometer (Advion Inc., Ithaca, NY, USA). High-resolution, accurate mass measurements were performed using the ExactiveTM Plus Orbitrap mass spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA).

General Procedure for the Synthesis of Schiff Bases (1-17)
To a solution of an appropriate aldehyde (1 EQ) in dry dichloromethane or dry methanol was added an amine (1 EQ). The resultant solution was stirred for 15 min before Na 2 SO 4 (4 EQ) was added. The reaction mixture was then stirred at room temperature until TLC showed complete consumption of the starting material (30 min to 16 h). Next, the drying agent was removed by filtration, and the volatiles were removed under reduced pressure to afford the desired products, which were used in the next step without further purification.
N-Benzyl-3-methylbutan-1-imine (15), quantitative yield, light orange oil. 1 (18) N-phthaloylglycine (2.00 g, 9.75 mmol, 1 EQ) was dissolved in dry dichloromethane (10 mL), and the solution was cooled to 0 • C using an ice bath before oxalyl chloride (0.95 mL, 10.73 mmol, 1.1 EQ) was added dropwise over 30 min. Upon complete addition, the reaction mixture was stirred at 0 • C for an additional 2 h, and the solvent was removed under reduced pressure without heating. The acyl chlorides thus obtained were used in the subsequent step without further purification.

General Procedure for the Synthesis of Diazoketone (31)
N-Benzyloxycarbonylglycine (2.09 g, 10.0 mmol, 1 EQ) was dissolved in dry tetrahydrofuran (20 mL), and the resultant solution was cooled to −20 • C using a sodium chloride ice bath before triethylamine (1.39 mL, 10.0 mmol, 1 EQ) was added in one portion. Ethyl chloroformate (1.91 mL, 10.0 mmol, 1 EQ) was then added dropwise, and the reaction mixture was stirred for another 1 h. The white precipitate formed was removed by filtration. To the filtrate were slowly added dry acetonitrile (80 mL) (4:1 solution in THF) and (trimethylsilyl)diazomethane (2.0 M solution in hexane, 10 mL, 20.0 mmol, 2 EQ). The resultant reaction mixture was then stirred at 4 • C for 24-48 h. The reaction was quenched by the addition of diethyl ether and 10% (m/m) aqueous citric acid. The organic phase was then washed with saturated aqueous NaHCO 3 and brine. The organic layer was dried over Na 2 SO 4, and the solvents were evaporated. The diazoketone was purified by silica gel column chromatography using EtOAc:Hex = 1:1, v/v as eluent.

General Procedure for the Synthesis of Mixed Anhydride
In a flame-dried flask, N-(tert-butoxycarbonyl)glycine (3.00 g, 17.13 mmol, 1 EQ) was dissolved in dry tetrahydrofuran (20 mL) and placed under an argon atmosphere. The solution was cooled to −60 • C, and triethylamine (2.62 mL, 18.84 mmol, 1.1 EQ) was added in one portion. Then ethyl chloroformate (2.13 mL, 22.27 mmol, 1.3EQ) was added dropwise over a period of 30 min. After the complete addition of the reagent, the reaction mixture was stirred at −40 • C for another 2 h. The resultant reaction mixture was then directly used in the next step without any further purification. The same reaction conditions were used for the synthesis of 2-(((benzyloxy)carbonyl)amino)acetic anhydride from ((benzyloxy)carbonyl)glycine.

General Procedure for the Synthesis of Monocyclic Beta Lactam Core I (19-30)
Schiff base (1 EQ) was dissolved in dry toluene (0.1-0.2 mmol/mL) in a flame-dried flask and placed under an argon atmosphere. Triethylamine (2.5 EQ) was then added in one portion, and the resultant solution was heated to 80 • C, before 2-(1,3-dioxoisoindolin-2-yl)acetyl chloride (1.3 EQ), dissolved in in dry toluene, was added dropwise over a period of 30 min. Upon complete addition, the reaction was stirred at 80 • C for a further 1.5-3.5 h. The reaction mixture was then cooled to room temperature, and the volatiles were removed in vacuo. The solid residue thus obtained was redissolved in ethyl acetate. The organic phase was washed with 10% aq. citric acid solution, saturated NaHCO 3, and brine. The organic phase was dried (Na 2 SO 4 ), filtered, then concentrated in vacuo. Some cyclized ß-lactams were purified by silica gel column chromatography using EtOAc: Hex as eluent.